Magnetic Effects of Electric Current((Term i)

8/12/2019 Magnetic Effects of Electric Current((Term i)

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MAGNETIC EFFECTS OF ELECTRIC CURRENT

Why does a compass needle get deflected?Compass needle get deflected when brought near magnet due to the force of the magnet.

The lines along which the iron fillings align themselves represent magnetic lines of force.

Q: What is a magnet?

A: Magnet :A substance which has the property of attracting other substances like iron filings and points in the

north south direction when suspended freely is called a magnet. So, a magnet is an object that attracts the pieces o

iron ,steel , nickel and cobalt.

Generally naturally occurring magnets are available in two types (i) Bar magnet (ii) Horse shoe magnet. The end of the bar magnets are known as pole. Magnetic field:The space around a magnet in which the force of attraction and repulsion due to it can be

detected is called the magnetic field. So, It is the space around the magnet where its presence can be felt.

Magnetic field lines:The curved paths along which the north pole of the compass needle moves in amagnetic field are called magnetic field lines. Magnetic field lines are used to represent a magnetic field. magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move.

Magnetic field is a quantity that has both magnitude and direction. The direction of the magnetic fielditaken to be the direction in which a north pole of the compass needle moves inside it. It is a vector quant

Magnetic field around a magnet can be detected by using a magnetic compass. The magnetic field lines around a magnet can be observed by sprinkling iron filings around a magnet. It

also be observed by moving a magnetic compass around a magnet.

The lines are more closer at the ends of the magnet and widely separated at other places i.e., it meansstrength of the magnet field is higher at poles and weaker in other places.

Figure 1

Q: Why magnetic field lines never intersect each other?A: The field lines emerge from the north poleand merge at the south pole out sidemagnet. Inside the magnet

direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curvethe point of intersection of the magnetic field lines the compass will have two directions of magnetic field lines and

impossible to have two direction of fields at a single point. This is why no two field-lines are found to cross e

other.


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Properties of magnetic field lines:

1. Magnet field lines start from the north pole of a magnet & end at the south pole of a magnet.2. The magnetic field lines are closed and continuous curves.3. Closer near the poles4. These lines never intersect each other .5. A magnetic compass, when placed at any of the points on a magnetic line, aligns itself along the tangen

the line of force at that point.

Figure 2

Figure 3

EARTHS MAGNETIC FIELD:

Earth consists of huge magnetic field .

The north pole of the magnet lies at the geographical south pole of the earth and south pole of the magnelies at the geographical North pole of the earth.

The axis of the earths magnetic field is inclined at an angle of about 15 degree with the geographical axis
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Q: What is a compass?

A: A compass is actually a magnetic needle having freely suspended magnet under it . which always point toward

north pole and south pole of the earth.

Compass needle:A compass needle is a small bar magnet whose ends always point towards north southdirection. The end pointing towards north is called North Pole and the end pointing towards south is called South

Pole.

Figure 4

Q: What is North pole and South pole of magnet?A: In a freely suspended magnet the end , pointing toward north direction of the earth is known as north pole andend pointing toward south pole of the earth is known as south pole of the magnet.

Oersteds Experiment: - In 1820 Oersted found that a current carrying conductor is able to deflect the compass

needle , it means a current carrying conductor have magnetic field around it . Take a magnetic needle S-N,

which can rotate freely about a vertical axis in a horizontal plane. Hold a conducting wire AB over the magnetic

needle NS parallel to it. Complete the circuit by closing the key, such that the current flows from B to A.It will be found that N-pole of the magnetic needle gets deflected towards west (fig.i). If the direction of the current is

reversed, the N-pole of magnetic needle deflected towards east (fig.ii). On increasing the current, the deflection of the

needle increases. This experiment shows that the magnetic field is only due to electric current. If a strong current ispassed through a conductor then magnetic field produced around the conductor is also very strong and the earthsmagnetic field may be neglected in its comparison. In such a case, magnetic lines of force around a conductor are circu

The direction of magnetic field is given by the following two rules.1. Amperes swimming rule: - If we imagine a man swimming along the wire in the direction of the current with

arms out stretched and facing the needle, the N-pole of the needle will be deflected towards his left hand.

2. Snow rule: - Snow rule illustrates that when current flows from S to N, the N-pole deflects towards west. If th

direction of the current is reversed the N-Pole will deflect towards east.3. Maxwells cork rule: - Imagine a right handed cork screw being driven along the wire in the direction of the

current. The direction in which the thumb rotates gives the direction of the N-pole of the needle.

Conclusions from Oeresteds experiment: -1. Whenever the current is passed through a straight conductor, it behaves like a magnet.

2. The magnitude of the magnetic effect increases with the strength of the current.

3. The magnetic field set up by the conductor is at right angles to the direction of the flow of the current.

4. The direction in which the N-pole of the magnetic needle will move depends upon:

i) direction of the current in the conductor

ii) the relative position of the conductor i.e., whether the conductor is above the needle or below the needle.


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5.This led to the discovery of magnetic effect of current.

MAXWELLS RIGHT HAND GRIP (THUMB) RULE

It says If the current carrying conductor wire is gripped with the right hand in such a way that the thumb po

towards the direction of the current, the direction of the curled fingers give the direction of the magnetic fproduced around the conductor wire.

Figure 5

Fig1: right hand rule


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Magnetic field due to current carrying conductor

The direction of magnetic field produced by the electric current depends upon the direction of flow of current. If

reverse the direction of current then the direction of magnetic field produced by the electric current get changed.

Figure 6

The magnitude of the magnetic field produced at a given point increases as the current through the wire increases

The concentric circles representing the magnetic field around a current-carrying straight wire become larger andlarger as we move away from it.

If a magnetic compass is placed near a conductor carrying current (wire), the needle is deflected. This shows that

conductor carrying current has a magnetic field around it.

If the direction of the current is from north to south, the deflection of the magnetic needle is towards the east.

If the direction of the current is from south to north, the deflection of the needle is towards the west.

The magnetic field around a current carrying straight conductor is in concentric circles. It can be observed by

passing a current carrying straight conductor through a cardboard and sprinkling iron filings on it.

Factors affecting strength of magnetic field around a current carrying straight conductor.Strength of

magnetic field is directly proportional to the current passing through the conductor and inversely proportional to

distance from the conductor. ( B I and B 1/r ) .

Properties of Magnetic lines of forces around the straight conductor: -

Or

Magnetic field due to straight current carrying wire:

1. The magnetic lines of forces are in the form of concentric circles near the conductor.2. On changing the direction of the current, the direction of the magnitude lines also reverses3. On increasing the magnitude of the current, the magnetic lines of forces increase.4. The plane magnetic lines of force is at right angles to the plane of the conductor carry

conductor.

5. The circles are more closer near the wire (this shows that field strength is more near wire).
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6. The direction of magnetic field due to straight current carrying wire is given by Maxweright hand thumb rule.

7. The magnetic field around a straight current carrying conductor is circular.

Magnetic field due to current carrying circular loop:

Figure 7

We know that the magnetic field produced by a current- carrying straight wire depends inversely on the distance.

We know that the magnetic field produced by a current- carrying conductor at a given point, depends directly on current passing through it.

Therefore, if there is a circular coil having n turns, the field produced is n times as large as produced by a single t

This is because the current in each circular turn has the same direction, and the field due to each turn then just adup.

When current is passed through a circular conductor (loop) the magnetic field produced is in the form of concen

circles around the conductor. Towards the centre the arcs of the circles become larger and appears as straight line

Magnetic field due to current carrying circular loop or coil :When the current is passed through circular looor coil, the lines of force are circular near the wire but straight and parallel near the centre of loop or coil.

Factors affecting magnetic field due to current carrying circular loop or coil.

Magnetic field due to current carrying circular loop at its centre is

Directly proportional to the current passing through it. Inversely proportional to the radius of loop.


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Figure 8

fig 4: field due to circular coil.

Factors on which the magnetic field produced by a current carrying circular coil depends:-The strength of t

magnetic field produced by a current carrying circular coil depends on the following factors:

i) Current flowing through the coil:- The strength of the magnetic field (B) is proportional to the current (I)

flowing through the coil.

Mathematically B I

ii) Radius of the coil: - The strength of the magnetic field(B) is inversely proportional to the radius (r) of thecircular coil.

Mathematically,

1


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B ------R

iii) Number of turns of wire in the circular coil:- The strength of the magnetic field(B) is proportional to the

number of turns of wire(n) in the circular coil

Mathematically, B n

Properties of magnetic lines of force around circular coil: -

1. Magnetic lines of forces are circular around the points where the current enters or leaves the circular coil.

2. Within the space enclosed by the coil, the magnetic lines of forces are in same direction.

3. Near the centre of the oil, the magnetic lines of forces are parallel. When the magnetic lines of forces are paral

the magnetic field is said to be uniform.

4. The magnetic lines of forces are at right angles to the plane of the coil.

5. The strength of the magnetic field is directly proportional to the current.

A SOLENOID

Magnetic field due to current in a solenoid:-

A solenoid is a circular coil of wire in the shape of a cylinder.

When current flows through a solenoid, it behaves like a bar magnet. The ends of the solenoid behaves like the

North and South poles of a magnet. The magnetic field produced by a solenoid is similar to the magnetic field

produced by a bar magnet.

The strength of the magnetic field depends upon the strength of the current and the number of turns of the coil.

A coil of many circular turns of insulted copper wire wrapped closely in the shape of a cylinder is called a soleno

So, the solenoid is a coil containing large number of close turns of insulated copper wires.

Its field pattern is similar to the pattern of bar magnet. Factors affecting the field strength of solenoid.(I) The number of turns of copper coil : more the turn more is the strength

(II) The strength of current : more the strength of current more is the strength of field.

Factors affecting Magnetic field due to current in a solenoid1. Magnetic field is directly proportional to the number of turns in the coil.

2. It is directly proportional to the current passing through it.

3. It is inversely proportional to the length of air gaps between the poles.

4. It depends on the nature of the core material used in the solenoid. Electromagnet.


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Figure 9

fig 2: field due to solenoid

5) Electromagnet:- A strong magnetic field inside a solenoid can be used to magnetise a piece of magnetic mate

like a soft iron when placed inside the coil. Such a magnet is called an Electromagnet. So, an electromagnet cons

of a long coil of insulated copper wires wound on soft iron core .If electric current is passed through a wire wound around a piece of soft iron, it behaves like a magnet. Such amagnet is called an electromagnet.

An electromagnet consists of a long coil of insulated copper wire wrapped around a soft iron core. It is a tempora

magnet as it works as long as current is passed through it. Factors affecting the strength of magnetic field of anelectromagnet.

Factors affecting the field strength of the electromagnet :

(i) The number of turns of the coil

(ii) The current flowing through the conductor


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(iii) The length of the air gap between its pole.

Q: Why soft iron Core is used in case of electromagnet ?

A: the core is made up of the soft iron because soft iron loses all magnetism when current in the coil is switched off .

The strength of magnetic field of an electromagnet is

1. Directly proportional to the number of turns.

2. Directly proportional to the current flowing through it.

3. Inversely proportional to the length of air gaps between the poles.

Uses of electromagnet

1. They are used in electrical devices such as electric bell, electric fan, motor, and generator.

2. They are used for lifting and transporting large mass of iron.

3. They are used in medical practices for removing pieces of iron from wound and used in MRI.

Permanent magnetsA permanent magnet is made from steel alloys like carbon steel, chromium steel, cobalt steel, etc. They are weak

than electromagnets and their strength and polarity cannot be changed. Force on a current carrying conductor in a

magnetic field.

Force on a conductor carrying current in a magnetic field

A.M.Ampere suggested that if a current carrying conductor produces a magnetic field and exerts a force on a

magnet, then a magnet should also exerts a force on a current carrying conductor.

Eg :- If an aluminium rod is suspended horizontally by a wire between the poles of a horse shoe magnet and curreis passed through the wire, then the aluminium rod is displaced. If the direction of current is reversed, the directioof displacement is also reversed. The force exerted is maximum if the conductor is perpendicular to the magnetic

field.

Factors on which the field depend

The strength of the magnetic field produced by a straight current carrying conductor depends on the following factors

i) Current passing through the conductor:- The strength of the magnetic field (B) is directly proportional to the cur

(I) passing through the conductor.

Mathematically B I.(i)

ii) Distance from the conductor:-

The strength of the magnetic field (B) is inversely proportional to the distance (r) from the conductor.

1

Mathematically, B ------- .(ii)r

Combining (i) and (ii), we getI

B --------

r

0I

or B = ---------- , where o is the permeability of vacuum.


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2r

A current carrying conductor placed in a magnetic field experiences a force due to the interaction betweena. Magnetic field due to current carrying conductor and

b. External magnetic field in which conductor is placed.

Force on a current carrying conductor in a magnetic fieldA current carrying conductor placed in a magnetic field experiences a force due to the interaction between

a. Magnetic field due to current carrying conductor and

b. External magnetic field in which conductor is placed.

Flemings Left Hand Rule

The direction of force (motion) of a current carrying conductor in a magnetic field is given by Flemings Left Ha

Rule.

It states that If we hold the thumb, fore finger and middle finger of the left hand perpendicular to each other suc

that the fore finger points in the direction of magnetic field, the middle finger points in the direction of current, th

the thumb shows the direction of force (motion) of the conductor.

Figure 10

7) Electromagnetic induction:- (Michael Faraday )

The motion of a magnet with respect to a coil or a change in the magnetic field induce a potential difference in thcoil and produces induced current. This is called electromagnetic induction.

i) Motion of a magnet with respect to a coil produces induced current :-

If a magnet is moved towards or away from a coil of wire connected to a galvanometer, the galvanometer needle

shows a deflection. This shows that current is induced in the coil due to the motion of the magnet.ii) Change in magnetic field produces induced current :

Take two coils of wires wound around a cylindrical paper roll. Connect one coil to a battery and the other coil to

galvanometer. If current is passed through the first coil, the galvanometer needle shows a deflection in the second

coil. If the current is disconnected, the needle moves in the opposite direction. This shows that current is induced


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due to change in magnetic field.

Electric motor

A motor is the device which converts electrical energy into mechanical energy. It has a shaft which rotates

continuously when current is passed through it. It is used in electric fans, mixer grinder, etc.

Principle of electric motor

When a rectangular coil of copper wire is placed in a magnetic field and current is passed through it, a force acts the coil which rotates it continuously. Electromagnetic induction. The production of electric current by moving a

straight conductor in a magnetic field is called electromagnetic induction. It is the production of electricity from

magnetism.

Flemings Right Hand RuleThe direction of induced current is given by Flemings RightHand Rule.

It states that If the thumb, fore finger and middle finger of the right hand is held perpendicular to each other sucthat the thumb points in the direction of motion of the conductor, the fore finger points in the direction of the

magnetic field, then the middle finger shows the direction of induced current .

Figure 11

8) Direct and Alternating current:-

a) Direct current (DC) :-

A current that always flows in one direction only is called direct current.

The current we get from a battery is a direct current.

b) Alternating current (AC) :-

A current that reverses its direction periodically is called alternating current.

Most power stations in our country produce alternating current. AC changes direction every 1/100 second and its

frequency is 50 Hertz (Hz).

One advantage of AC over DC is that it can be transmitted over long distances without much loss of energy.

9) Domestic electric circuit:-

Electric power to homes is supplied through the mains. It has two wires. One is a live wire (positve wire) with red

insulation and the other is a neutral wire (negative wire) with black insulation. The potential difference between t

two wires is 220V. The earth wire with green insulation is connected to a metal plate kept in the ground.

Two separate circuits are used. One is of 15A for appliances with high power rating like gysers, air conditioners e

The other is of 5A for fans, bulbs etc. The different appliances are connected in parallel so that every appliance g

equal voltage and even if one is switched off the others are not affected.


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SOME IMPORTANT TERMS

1. Electric Motor: Converts electric current into mechanical energy

2. Generator: Converts mechanical energy into electrical current.

3. Fuse : Most important safety device, used for protecting circuits due to short-circuiting/ overloading of current

4. Earth wire: Also called green-insulator converted to a metallic body deep inside earth.

5. Electromagnetic induction : Phenomenon that produces induced current in a coil placed in a region where themagnetic field changes with time.

Figure 12

Figure 13


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Figure 14

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Magnetic Effects of Electric Current((Term i)

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